Geochemistry and origin of ferruginous nodules in weathered granodioritic gneisses, Mysore Plateau, Southern India Jayant K. Tripathi * , V. Rajamani National Facility for Geochemical Research, School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India Received 23 November 2005; accepted in revised form 3 January 2007; available online 10 January 2007 Abstract Fe-nodules occur within saprolites formed from weathering of granodioritic gneisses in the rain-shadow region of the Mysore Plateau adjacent to the Sahyadri Mountains in Southern India. These nodules and their host saprolites were studied for their geochemistry, including chemical speciation, to understand nodule formation and chemical redistribution processes during rock weathering. From their mode of occurrence, and mineralogical and geochemical data, we infer that the nodules originated by a two-stage process in which the initial extensive weathering of gneisses likely facilitated subsequent ferrolysis weathering and nodule formation. Nodules originated by precipitation of goethite, hematite and gibbsite along with several amorphous phases within the matrix of weathered gneisses. This is possible only under hydromorphic conditions, suggesting that parts of the plateau must have gone through a humid phase prior to the present aridity. In the saprolites, Al, Fe, and Ti become enriched because of the removal of Si, Ca, Na, and K. However within the nodule, Fe, Ti, Cr, and Ni are deposited after their chemical transport from the saprolite. Titanium, known for its immobile nature, was also mobilized and concen- trated under the conditions of nodule formation. The most important elements in the nodule constitution are Fe, Al, Ti, and Mn, each having both crystalline and amorphous phases. Fe–Ti and Mn oxyhydroxides grain coatings in the saprolites and discrete amorphous Mn and Ti phases in the nodules seem to have scavenged trace elements from the weathering profile. REE were mobilized during weathering and nodule genesis in which Ce and Ti show a strong geochemical coherence. The enrich- ment of only HREE in saprolite, and both HREE and LREE with significant Ce in the nodule, indicate the control of evolv- ing secondary minerals in the REE redistribution during rock weathering. Strong enrichment of Ce in the weathering profile and in nodules has important implications to the REE chemistry of river waters. Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Fe–Mn nodules occur on the ocean floor at all latitudes (Crerar and Barns, 1974); continental soil nodules, howev- er, are less abundant and only occur in the fine-textured soils of warm humid areas. The Fe content of these soil nodules has been shown to increase with mean annual pre- cipitation (Stiles et al., 2001). There has been considerable interest and discussion on the origins of oceanic and continental nodules based on their geochemistry. Oceanic nodules are thought to form by adsorption and catalytic oxidation of Fe and Mn at nucleating surfaces, and grow through autocatalysis (Palumbo et al., 2001). Marine bacte- ria also seem to play an important role in accelerating the rate of precipitation and nodule formation (Crerar and Barns, 1974). In soils, seasonal changes in soil redox poten- tial and pH are known to cause precipitation of nodule minerals in soil/sediment pore spaces (Burns and Burns, 1975; Palumbo et al., 2001 and references therein). Here again, precipitation of nodule minerals is thought to be microbially mediated (Spilde et al., 2002). Repetitive disso- lution of Fe and Mn during wet periods and precipitation during dry periods often produces concentric nodule layer- ing due to seasonal growth (Manceau et al., 2003). Nahon (1991) suggested that epigenetic replacement of kaolinite 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.01.001 * Corresponding author. Present address: Wadia Institute of Himalayan Geology, 33, GMS Road, Dehradun 248001, India. E-mail address: jktrip@yahoo.com (J.K. Tripathi). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 71 (2007) 1674–1688